Acl accommodating tibial design

ABSTRACT

Surgical methods and tibial implants ( 46 ) for accommodating the anterior cruciate ligament (ACL) during unicompartmental or bi-unicompartmental knee arthroplasty procedures.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication No. 61/076,206, entitled “ACL Accommodating Tibial Design,”filed on Jun. 27, 2008 by the same inventors hereof, the disclosure ofwhich is expressly incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to knee arthroplasty, and, moreparticularly, to surgical methods and a tibial implant for accommodatingthe anterior cruciate ligament during unicompartmental orbi-unicompartmental knee arthroplasty procedures.

2. Description of the Related Art

Unicompartmental knee arthroplasty and bi-unicompartmental kneearthroplasty procedures involve replacing damaged articular surfaces ofa knee joint, while retaining at least the central compartment and thecruciate ligaments. The benefits of both compartmental proceduresinclude the availability of minimally invasive procedures, quickerrecovery, and retained knee kinematics compared with total kneearthroplasty procedures.

During a unicompartmental knee arthroplasty procedure, repairs may bemade to either the medial compartment or the lateral compartment of theknee, while retaining the other side compartment, the centralcompartment, and the cruciate ligaments. This procedure requiresresection of either the medial compartment or the lateral compartment.

During a bi-unicompartmental knee arthroplasty procedure, repairs may bemade to both the medial compartment and the lateral compartment, whilestill retaining most of the central compartment and the cruciateligaments. This procedure requires resection of both the medial andlateral compartments. Also, in some cases, a portion of the centralcompartment is resected to bridge the resected medial and lateralcompartments.

The knee joint includes the anterior cruciate ligament (ACL) and theposterior cruciate ligament (PCL). The ACL controls hyperextension ofthe knee. As the knee joint is positioned in extension, the ACL tightensand functions to prevent hyperextension of the knee. The ACL runs fromthe posterior side of the femur to the anterior side of the tibia. Morespecifically, the ACL runs from the posterior side of the femur to aspinous process on the anterior side of the tibia. The spinous processis primarily centered on the proximal tibia, but it also extends intothe medial and lateral compartments of the proximal tibia.

Current resection methods often result in removing a significant portionof the spinous process to which the ACL attaches. Generally, enough ofthe spinous process remains to maintain ACL attachment. However, thestrength of the remaining spinous process is significantly undermined,so much so that when the ACL of a knee arthroplasty patient tightens,the spinous process may crack or even break away from the tibia.

SUMMARY

The present invention provides surgical methods for preparing a tibiafor a unicompartmental or bi-unicompartmental knee arthroplastyprocedure to accommodate the ACL. The tibia includes an articularsurface that interacts with a femur, a posterior edge, an anterior edge,a lateral edge, a medial edge, a central compartment, a lateralcompartment, and a medial compartment. The tibia also includes alongitudinal axis located within a dividing plane, where the dividingplane extends parallel to a sagittal plane from the posterior edge tothe anterior edge of the tibia. The tibia further includes a spinousprocess to which the ACL attaches. Accommodating the ACL involvespreserving the spinous process, and in turn, preserving the naturalfixation between the ACL and the spinous process without significantlyincreasing the risk of damage to the spinous process.

According to an embodiment of the present invention, the method involvesmaking a sagittal cut in the articular surface of the tibia. Thesagittal cut extends from the posterior edge to the anterior edge of thetibia, and at least a portion of the sagittal cut extends obliquely tothe dividing plane, such that the obliquely extending portion of thesagittal cut extends away from the central compartment of the tibia. Themethod also involves making a transverse cut beneath the articularsurface of the tibia. The transverse cut extends from either a lateralor medial edge of the tibia to the sagittal cut and generallytransversely to a longitudinal axis of the tibia. A cut edge formsbetween the transverse cut and the sagittal cut. The method furtherinvolves resecting either a medial or lateral compartment of the tibiaalong the cut edge, while retaining at least a portion of a centralcompartment of the tibia and at least a portion of the spinous processto which the ACL attaches.

According to another embodiment of the present invention, the methodinvolves making a sagittal cut in the articular surface of the tibia.The sagittal cut extends from the posterior edge to the anterior edge ofthe tibia. The method also involves making a transverse cut beneath thearticular surface of the tibia. The transverse cut extends from either alateral or medial edge of the tibia to the sagittal cut and generallytransversely to a longitudinal axis of the tibia. A chamfered cut edgeforms between the transverse cut and the sagittal cut. The methodfurther involves resecting either a medial or lateral compartment of thetibia along the chamfered cut edge, while retaining at least a portionof a central compartment of the tibia and at least a portion of thespinous process to which the ACL attaches.

The present invention also provides implants for accommodating the ACLduring a unicompartmental or bi-unicompartmental knee arthroplastyprocedure. The implant includes a body. The implant may further includea bearing layer.

According to an embodiment of the present invention, the body includes aposterior edge, an anterior edge, an inner edge, either a lateral edgeor a medial edge, a top surface, and a bottom surface. The posterior,anterior, inner, and lateral or medial edges span the top and bottomsurfaces of the body. The inner edge of the body corresponds to thesagittal cut and the cut edge of the tibia, where at least a portion ofthe sagittal cut extends obliquely to the dividing plane.

According to another embodiment of the present invention, the bodyincludes a posterior edge, an anterior edge, a lateral edge, a medialedge, a first inner edge, a second inner edge, a bridge having an inneredge, a top surface, and a bottom surface. The bridge connects the firstand second inner edges. The bridge, the first and second inner edges,and the posterior, anterior, lateral, and medial edges span the top andbottom surfaces of the body. In one form thereof, the bridge inner edgeis at least partially chamfered. In another form thereof, the bridgeinner edge approaches the anterior edge of the body between the firstinner edge and the second inner edge.

By retaining more of the spinous process during a unicompartmental orbi-unicompartmental knee arthroplasty procedure, there is provided agreater surface for ACL attachment. Also, the strength of the spinousprocess may be greatly improved. This improved strength may avoid thecracking and the breaking away of the spinous process currently seenwhen the ACL tightens and pulls upon a weakened spinous process. Still,enough of the tibia may be resected to support an implant and the loadupon that implant.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescriptions of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a plan view of the proximal tibia illustrating a surgicalmethod of the present invention;

FIG. 2 is a plan view of the proximal tibia illustrating other surgicalmethods of the present invention;

FIG. 3 is a plan view of the proximal tibia illustrating still othersurgical methods of the present invention;

FIG. 4 is a plan view of an implant of the present invention attached tothe proximal tibia;

FIG. 5 is a cross-sectional view of the implant of FIG. 4, taken alongline 5-5 of FIG. 4;

FIG. 6 is a plan view of another implant of the present invention;

FIG. 7 is a cross-sectional view similar to FIG. 5 of another implant ofthe present invention;

FIG. 8 is a cross-sectional view similar to FIG. 5 of still anotherimplant of the present invention;

FIG. 9 is a plan view of the implant of FIG. 6 attached to the proximaltibia;

FIG. 10 is a plan view of another implant of the present invention;

FIG. 11 is cross-sectional view of the implant of FIG. 10, taken alongline 11-11 of FIG. 10;

FIG. 12 is cross-sectional view of the implant of FIG. 10, taken alongline 12-12 of FIG. 10;

FIG. 13 is an elevational view of the proximal tibia illustrating othersurgical methods of the present invention; and

FIG. 14 is a plan view of the proximal tibia of FIG. 14.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate preferred embodiments of the invention and suchexemplifications are not to be construed as limiting the scope of theinvention any manner.

DETAILED DESCRIPTION

Referring to FIGS. 1-3 and 13-14, tibia 10 of a right leg has articularsurface 12 that is configured to articulate with a femur (not shown).Tibia 10 also includes posterior edge 14, anterior edge 16, lateral edge18, and medial edge 20. Tibia 10 further includes central compartment22, lateral compartment 24, and medial compartment 26.

As shown in FIG. 1, spinous process 28 is located primarily in centralcompartment 22 of tibia 10, but may also extend into lateral compartment24 and medial compartment 26 of tibia 10. Spinous process 28 is a bonyprominence of tibia 10 to which the ACL (not shown) attaches.

As shown in FIGS. 13 and 14, tibia 10 includes longitudinal axis 32.Longitudinal axis 32 is located within dividing plane 30 which runsthrough tibia 10 in an anterior/posterior direction from anterior edge16 to posterior edge 14. Dividing plane 30 is substantially parallel toa sagittal plane of the body and is substantially transverse to ahorizontal plane of the body.

To prepare tibia 10 for a unicompartmental or bi-unicompartmental kneearthroplasty procedure, either lateral compartment 24 or medialcompartment 26, or both, must be resected. The following descriptionsmay pertain to lateral compartment 24, but the same principles may beapplied to medial compartment 26. Resection of lateral compartment 24involves making sagittal cut 34 and transverse cut 36 into tibia 10. Asshown in FIG. 14, sagittal cut 34 is made into articular surface 12 oftibia 10 from posterior edge 14 to anterior edge 16 and between centralcompartment 22 and lateral compartment 24. As shown in FIG. 13,transverse cut 36 is made beneath articular surface 12 from lateral edge18 to sagittal cut 34. Transverse cut 36 extends generally transverselyto longitudinal axis 32 of tibia 10 and generally parallel to thehorizontal plane. A surgeon may use a reciprocating saw and a cuttingguide (not shown), a mill and a mill guide (not shown), or anothersuitable surgical tool to make sagittal cut 34 and transverse cut 36 intibia 10. For example, when using a mill having a rotating burr, a sideedge of the burr may form sagittal cut 34 in tibia 10 and a bottom edgeof the burr may form transverse cut 36 in tibia 10. Sagittal cut 34 andtransverse cut 36 intersect at cut edge 38. After sagittal cut 34 andtransverse cut 36 are made, lateral compartment 24 may be resected fromtibia 10 along cut edge 38.

According to a first embodiment of the present invention, as shown inFIGS. 1 and 13-14, sagittal cut 34 may extend substantially parallel todividing plane 30 from posterior edge 14 to anterior edge 16. Toaccommodate spinous process 28, cut edge 38 between sagittal cut 34 andtransverse cut 36 may be at least partially chamfered. In an exemplaryembodiment of the present invention, cut edge 38 is at least partiallychamfered in the area adjacent to spinous process 28. For purposes ofthis disclosure, cut edge 38 is “chamfered” whenever sagittal cut 34 andtransverse cut 36 intersect to form an angle that is greater than 90degrees. For example, as illustrated in FIG. 5, a chamfered cut edge 38may be formed between sagittal cut 34 that is angled away fromlongitudinal axis 32 and an essentially horizontal transverse cut 36.For purposes of this disclosure, cut edge 38 is also “chamfered”whenever sagittal cut 34 and transverse cut 36 intersect to form arounded or curved cut edge 38, as illustrated in FIGS. 7 and 13.Compared to current resection methods, both chamfering techniques retainmore bone at the base of spinous process 28 and avoid under-cuttingspinous process 28. Cut edge 38 may be chamfered by using, for example,a cutting guide (not shown) that matches the desired shapes of sagittalcut 34 and transverse cut 36.

According to another embodiment of the present invention, to furtheraccommodate spinous process 28, sagittal cut 34 may deviate from a pathessentially parallel to dividing plane 30. In other words, at least aportion 40 of sagittal cut 34 may extend obliquely to dividing plane 30,thereby extending away from central compartment 22 and away from spinousprocess 28.

As shown in FIG. 2, sagittal cut 34 may extend substantially parallel todividing plane 30 from posterior edge 14 of tibia 10 to deflection point42 spaced between posterior edge 14 and anterior edge 16 of tibia 10.Between deflection point 42 and anterior edge 16 of tibia 10, sagittalcut 34 may extend obliquely to dividing plane 30. In one form thereof,illustrated in lateral compartment 24 of FIG. 2, obliquely extendingportion 40 may extend linearly between deflection point 42 and anterioredge 16 of tibia 10, forming acute angle 44 between dividing plane 30and obliquely extending portion 40, where acute angle 44 is greater than0 degrees. For example, acute angle 44 may be as small as approximately1 degree, 2 degrees, 3 degrees, 4 degrees, or 5 degrees or as large asapproximately 10 degrees, 15 degrees, 20 degrees, 25 degrees, 30degrees, or more. A single deflection point 42 located approximatelymidway between posterior edge 14 and anterior edge 16 of tibia 10 isillustrated in FIG. 2, but it is also within the scope of the presentinvention that deflection point 42 may be located nearer to eitherposterior edge 14 or anterior edge 16, and that there may be multipledeflection points 42. In that form, obliquely extending portion 40 mayinclude any number of substantially linear, non-collinear cuts thatintersect at multiple deflection points 42. In another form thereof,illustrated in medial compartment 26 of FIG. 2, obliquely extendingportion 40 may curve from deflection point 42 to anterior edge 16 oftibia 10.

As shown in FIG. 3, entire sagittal cut 34 may extend obliquely todividing plane 30 from posterior edge 14 to anterior edge 16 of tibia10. In one form thereof, illustrated in lateral compartment 24 of FIG.3, obliquely extending portion 40 of sagittal cut 34 may extend linearlyfrom posterior edge 14 to anterior edge 16 of tibia 10, forming acuteangle 44 between dividing plane 30 and obliquely extending portion 40,where acute angle 44 is greater than 0 degrees. For example, acute angle44 may be as small as approximately 1 degree, 2 degrees, 3 degrees, 4degrees, or 5 degrees or as large as approximately 10 degrees, 15degrees, 20 degrees, 25 degrees, 30 degrees, or more. In another formthereof, illustrated in medial compartment 26 of FIG. 3, obliquelyextending portion 40 may curve from posterior edge 14 to anterior edge16 of tibia 10.

According to yet another embodiment of the present invention, to evenfurther accommodate spinous process 28, portion 40 of sagittal cut 34may extend obliquely to dividing plane 30 and, in addition, cut edge 38may be at least partially chamfered. In an exemplary embodiment of thepresent invention, cut edge 38 is at least partially chamfered in thearea adjacent to spinous process 28. Both techniques are discussed inmore detail above.

Referring again to FIGS. 13 and 14, to prepare tibia 10 for abi-unicompartmental knee arthroplasty procedure, both lateralcompartment 24 and medial compartment 26 must be resected. The resectionof lateral compartment 24 and medial compartment 26 can be performed inaccordance with the teachings set forth above. Further, during abi-unicompartmental knee arthroplasty procedure, a portion of centralcompartment 22, referred to herein as bridge portion 78, may be resectedto join resected lateral compartment 24 and resected medial compartment26. Bridge portion 78 may be located in central compartment 22 nearanterior edge 16 of tibia 10. By resecting bridge portion 78, a singletibial implant 46 that spans resected lateral compartment 24, resectedbridge portion 78 of central compartment 22, and resected medialcompartment 26 can be utilized, as described in more detail below.

Referring still to FIGS. 13 and 14, resection of bridge portion 78 ofcentral compartment 22 involves making vertical bridge cut 80 andhorizontal bridge cut 82 into tibia 10. Vertical bridge cut 80 is madeinto articular surface 12 of tibia 10 and extends from sagittal cut 34in lateral compartment 24 to sagittal cut 34 in medial compartment 26.Horizontal bridge cut 82 is made beneath articular surface 12 andextends from anterior edge 16 of tibia to vertical bridge cut 80.Horizontal bridge cut 82 also extends from transverse cut 36 in lateralcompartment 24 to transverse cut 36 in medial compartment 26. Liketransverse cut 36, horizontal bridge cut 82 extends generallytransversely to longitudinal axis 32 of tibia 10 and generally parallelto the horizontal plane of the body. Vertical bridge cut 80 andhorizontal bridge cut 82 intersect at bridge cut edge 84. After verticalbridge cut 80 and horizontal bridge cut 82 are made, bridge portion 78of central compartment 22 may be resected from tibia 10 along bridge cutedge 84.

According to an embodiment of the present invention, to accommodatespinous process 28, bridge cut edge 84 may be at least partiallychamfered. Similar to the chamfering of cut edge 38, bridge cut edge 84is “chamfered” whenever vertical bridge cut 80 and horizontal bridge cut82 intersect to form an angle that is greater than 90 degrees. Bridgecut edge 84 is also “chamfered” whenever vertical bridge cut 80 andhorizontal bridge cut 82 intersect to form a rounded or curved bridgecut edge 84, as illustrated in FIGS. 13 and 14. Compared to currentresection methods, both chamfering techniques retain more bone at thebase of spinous process 28 and avoid under-cutting spinous process 28.

According to another embodiment of the present invention, to furtheraccommodate spinous process 28, vertical bridge cut 80 may extend awayfrom spinous process 28 and toward anterior edge 16 of tibia 10 betweenlateral compartment 24 and medial compartment 26 of tibia 10. Forexample, as illustrated in FIGS. 13-14, as vertical bridge cut 80approaches dividing plane 30, vertical bridge cut 80 curves towardanterior edge 16 of tibia 10. Vertical bridge cut 80 need not extendlinearly from lateral compartment 24 to medial compartment 26. Verticalbridge cut 80 may be shaped by, for example, milling tibia 10 or cuttingtibia 10 with a reciprocating saw blade.

According to yet another embodiment of the present invention, to evenfurther accommodate spinous process 28, vertical bridge cut 80 may curveaway from spinous process 28 and, in addition, bridge cut edge 84 may beat least partially chamfered. Both techniques are discussed in moredetail above.

Referring next to FIGS. 4-12, tibial implant 46 is provided for use withthe surgical methods set forth above. According to an exemplaryembodiment of the present invention, tibial implant 46 is contoured tomimic the size and shape of tibia 10 after tibia 10 is resected using atleast one of the surgical methods set forth above. Implant 46 mayinclude body 48 and bearing layer 50. As illustrated in FIGS. 4 and 9,body 48 of implant 46 is attached to tibia 10 of a right leg, but thesame principles discussed herein may be applied to a left leg.

Body 48 may be constructed of any biocompatible ceramic or metal,including, but not limited to, titanium, a titanium alloy, cobaltchromium, cobalt chromium molybdenum, porous tantalum, or a highlyporous biomaterial. A highly porous biomaterial is useful as a bonesubstitute and as cell and tissue receptive material.

A highly porous biomaterial may have a porosity as low as 55, 65, or 75percent and as high as 80, 85, or 90 percent. An example of such amaterial is produced using Trabecular Metal™ technology generallyavailable from Zimmer, Inc., of Warsaw, Ind. Trabecular Metal™ is atrademark of Zimmer Technology, Inc. Such a material may be formed froma reticulated vitreous carbon foam substrate which is infiltrated andcoated with a biocompatible metal, such as tantalum, by a chemical vapordeposition (“CVD”) process in the manner disclosed in detail in U.S.Pat. No. 5,282,861, the disclosure of which is expressly incorporatedherein by reference. In addition to tantalum, other metals such asniobium, or alloys of tantalum and niobium with one another or withother metals may also be used.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%-85% or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of body 48 of implant 46 to tibia 10.

The porous tantalum structure may be made in a variety of densities toselectively tailor the structure for particular applications. Inparticular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone to provide an improved matrix for bone ingrowthand mineralization.

In one form thereof, illustrated in FIGS. 4, 5, 7, and 8, body 48 hasfirst posterior edge 52, first anterior edge 54, first inner edge 56,either lateral edge 58 or medial edge 60, top surface 62, and bottomsurface 64. First posterior edge 52, first anterior edge 54, first inneredge 56, and either lateral edge 58 or medial edge 60 span top surface62 and bottom surface 64 of body 48. Body 48, as set forth in this firstembodiment, may be used during unicompartmental and bi-unicompartmentalknee arthroplasty procedures. During a unicompartmental kneearthroplasty procedure, a single body 48 may be implanted into eitherresected compartment of tibia 10—either lateral compartment 24 or medialcompartment 26. During a bi-unicompartmental knee arthroplastyprocedure, body 48 may be implanted into lateral compartment 24, andanother body 48 may be implanted into medial compartment 26, as shown inFIG. 4. Body 48 may be secured to lateral compartment 24 of tibia 10such that: bottom surface 64 of body 48 corresponds to transverse cut 36of tibia 10 in lateral compartment 24; first inner edge 56 of body 48corresponds to sagittal cut 34 and cut edge 38 of tibia 10 in lateralcompartment 24; first posterior edge 52 of body 48 corresponds toposterior edge 14 of tibia 10; first anterior edge 54 of body 48corresponds to anterior edge 16 of tibia 10; and lateral edge 58 of body48 corresponds to lateral edge 18 of tibia 10. Similarly, body 48 may besecured to medial compartment 26 of tibia 10 such that: bottom surface64 of body 48 corresponds to transverse cut 36 of tibia 10 in medialcompartment 26; first inner edge 56 of body 48 corresponds to sagittalcut 34 and cut edge 38 of tibia 10 in medial compartment 26; firstposterior edge 52 of body 48 corresponds to posterior edge 14 of tibia10; first anterior edge 54 of body 48 corresponds to anterior edge 16 oftibia 10; and medial edge 60 of body 48 corresponds to medial edge 20 oftibia 10.

In another form thereof, illustrated in FIGS. 6 and 9-12, body 48 hasboth first posterior edge 52 and second posterior edge 66, both firstanterior edge 54 and second anterior edge 68, both first inner edge 56and second inner edge 70, bridge 72 connecting first inner edge 56 andsecond inner edge 70, bridge 72 having its own bridge inner edge 73,both lateral edge 58 and medial edge 60, top surface 62, and bottomsurface 64. First and second posterior edges 52 and 66, first and secondanterior edges 54 and 68, first and second inner edges 56 and 70, bridge72, bridge inner edge 73, lateral edge 58, and medial edge 60 span topsurface 62 and bottom surface 64 of body 48. Bridge 72, which connectsfirst inner edge 56 and second inner edge 70, may also connect firstanterior edge 54 and second anterior edge 68. Body 48, as set forth inthis second embodiment, is designed for use during a bi-unicompartmentalknee arthroplasty procedure. Body 48 may be implanted into both resectedcompartments of tibia 10—both lateral compartment 24 and medialcompartment 26 —with bridge 72 spanning bridge portion 78 of centralcompartment 22. More specifically, body 48 may be secured to tibia 10such that: bottom surface 64 of body 48 corresponds to transverse cut 36in lateral compartment 24, transverse cut 36 in medial compartment 26,and horizontal bridge cut 82 in central compartment 22; first inner edge56 of body 48 corresponds to sagittal cut 34 and cut edge 38 in lateralcompartment 24; second inner edge 70 of body 48 corresponds to sagittalcut 34 and cut edge 38 in medial compartment 26; bridge inner edge 73 ofbody 48 corresponds to vertical bridge cut 80 and bridge cut edge 84 incentral compartment 22; first posterior edge 52 and second posterioredge 66 of body 48 correspond to posterior edge 14 of tibia 10; firstanterior edge 54 and second anterior edge 68 of body 48 correspond toanterior edge 16 of tibia 10; lateral edge 58 of body 48 corresponds tolateral edge 18 of tibia 10; and medial edge 60 of body 48 correspondsto medial edge 20 of tibia 10.

Body 48 may be secured to tibia 10 by any method known in the art. Anexemplary method of attachment involves using an adhesive, which may notonly provide a secure connection between body 48 and tibia 10, but mayalso strengthen tibia 10. The adhesive may be any known medical gradeadhesive having sufficient strength to secure body 48 to tibia 10,including, but not limited to, bone cement, light curable acrylicadhesives, acrylic adhesives, cyanoacrylate adhesives, siliconeadhesives, urethane adhesives, and epoxy adhesives.

According to an embodiment of the present invention, as shown in FIGS.4, 6, and 9, first inner edge 56 (and second inner edge 70 ifapplicable) of body 48 correspond to sagittal cut 34 of tibia 10, whereat least a portion 40 of sagittal cut 34 extends obliquely to dividingplane 30. Therefore, once implanted, at least a portion 74 of firstinner edge 56 (and second inner edge 70 if applicable) of body 48 alsoextends obliquely to dividing plane 30, thereby extending away fromcentral compartment 22 and away from spinous process 28.

According to another embodiment of the present invention, as shown inFIGS. 5, 7 and 11, first inner edge 56 (and second inner edge 70 ifapplicable) of body 48 correspond to cut edge 38 of tibia 10, where atleast a portion of cut edge 38 is chamfered. In other words, at least aportion of first inner edge 56 (and second inner edge 70 if applicable)of body 48 are also chamfered. For purposes of this disclosure, firstinner edge 56 and second inner edge 70 of body 48 are “chamfered”whenever first inner edge 56 or second inner edge 70 and bottom surface64 of body 48 form an angle that is greater than 90 degrees. Forexample, as illustrated in FIG. 5, first inner edge 56 of body 48 may beangled away from longitudinal axis 32. For purposes of this disclosure,first inner edge 56 and second inner edge 70 of body 48 are also“chamfered” whenever the edges are rounded, as illustrated in FIGS. 7and 11.

According to yet another embodiment of the present invention, bridgeinner edge 73 of body 48 corresponds to vertical bridge cut 80 of tibia10, where vertical bridge cut 80 extends away from spinous process 28and toward anterior edge 16 of tibia 10. For example, as shown in FIGS.6 and 9-10, bridge inner edge 73 of body 48 does not extend linearlybetween first inner edge 56 and second inner edge 70 of body 48. Rather,once implanted, bridge inner edge 73 curves away from spinous process 28and toward anterior edge 16 of tibia 10 near dividing plane 30.

According to still yet another embodiment of the present invention,bridge inner edge 73 of body 48 corresponds to bridge cut edge 84 oftibia 10, where at least a portion of bridge cut edge 84 is chamfered.In other words, at least a portion of bridge inner edge 73 of body 48 isalso chamfered. For purposes of this disclosure, bridge inner edge 73 ofbody 48 is “chamfered” whenever bridge inner edge 73 and bottom surface64 of body 48 form an angle that is greater than 90 degrees. Bridgeinner edge 73 of body 48 is also “chamfered” whenever the edge isrounded, as illustrated in FIG. 12.

As mentioned above, implant 46 may include bearing layer 50. Morespecifically, implant 46 may include bearing layer 50 attached to topsurface 62 of body 48. It is within the scope of the present inventionthat body 48 and bearing layer 50 of implant 46 may be integrally formedof a single material or more than one material. Bearing layer 50 isconfigured to interact with a femoral component (not shown).

Bearing layer 50 may be constructed of a polymer, including, but notlimited to, a poly ether ether ketone, fiber reinforced poly ether etherketone, ultrahigh molecular weight polyethylene, crosslinked ultrahighmolecular weight polyethylene, or polyether ketone ether ether ketone.Advantageously, by utilizing a rigid material for body 48, such as ametal, with a more resilient material for bearing layer 50, such as apolymer, wear of the femoral component (not shown) may be reduced.

According to yet another exemplary embodiment of the present invention,as shown in FIGS. 4-8, bearing layer 50 may extend peripherally at leastpartially beyond first inner edge 56 of body 48, second inner edge 70 ofbody 48, and/or bridge inner edge 73 of body 48, thereby forming bearingoverhang 76. Bearing overhang 76 may protect portions of articularsurface 12 and spinous process 28 that remain after resection. Bearingoverhang 76 is especially useful for resections performed with sagittalcut 34 that includes obliquely extending portion 40, because greaterportions of articular surface 12 and spinous process 28 remain, ascompared to resections performed with sagittal cut 34 that extendsessentially parallel to dividing plane 30. Bearing overhang 76 may bespaced away from tibia 10, or bearing overhang 76 may abut tibia 10.

While this invention has been described as having preferred designs, thepresent invention can be further modified within the spirit and scope ofthis disclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the invention using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

1. A method of preparing a tibia for knee arthroplasty to accommodatethe anterior cruciate ligament, the tibia having an articular surfacethat interacts with a femur, a posterior edge, an anterior edge, alateral edge, a medial edge, a central compartment, a lateralcompartment, a medial compartment, a longitudinal axis located within adividing plane, the dividing plane extending parallel to a sagittalplane and from the posterior edge to the anterior edge of the tibia, anda spinous process to which the anterior cruciate ligament attaches, themethod comprising the steps of: making a sagittal cut in the articularsurface of the tibia, the sagittal cut extending from the posterior edgeto the anterior edge of the tibia, at least a portion of the sagittalcut extending obliquely to the dividing plane, whereby the obliquelyextending portion of the sagittal cut extends away from the centralcompartment of the tibia; making a transverse cut beneath the articularsurface of the tibia, the transverse cut extending from one of thelateral edge of the tibia and the medial edge of the tibia to thesagittal cut and generally transversely to the longitudinal axis of thetibia, thereby forming a cut edge between the transverse cut and thesagittal cut; and resecting one of the medial and lateral compartmentsof the tibia along the cut edge, while retaining at least a portion ofthe central compartment of the tibia and at least a portion of thespinous process to which the anterior cruciate ligament attaches.
 2. Themethod of claim 1, wherein: the sagittal cut extends substantiallyparallel to the dividing plane from the posterior edge of the tibia to adeflection point spaced between the posterior edge and the anterior edgeof the tibia; and the sagittal cut extends obliquely to the dividingplane from the deflection point to the anterior edge of the tibia,whereby the obliquely extending portion of the sagittal cut forms anacute angle with the dividing plane.
 3. The method of claim 2, whereinthe acute angle formed between the sagittal cut and the dividing planeis between approximately 5 degrees and 30 degrees.
 4. The method ofclaim 2, wherein the acute angle formed between the sagittal cut and thedividing plane is approximately 15 degrees.
 5. The method of claim 1,wherein the sagittal cut extends obliquely to the dividing plane fromthe posterior edge to the anterior edge of the tibia, whereby thesagittal cut forms an acute angle with the dividing plane.
 6. The methodof claim 5, wherein the acute angle formed between the sagittal cut andthe dividing plane is between approximately 5 degrees and 30 degrees. 7.The method of claim 1, further comprising the step of attaching animplant to the resected compartment of the tibia, the implantcomprising: a body having a posterior edge, an anterior edge, an inneredge, one of a lateral edge and a medial edge, a top surface, and abottom surface, the posterior, anterior, inner, and one of the lateraland medial edges spanning the top and bottom surfaces of the body,wherein the inner edge of the body corresponds to the sagittal cut andto the cut edge of the tibia, one of the lateral and medial edges of thebody corresponds to one of the lateral and medial edges of the tibia,the posterior edge of the body corresponds to the posterior edge of thetibia, and the anterior edge of the body corresponds to the anterioredge of the tibia; and a bearing layer coupled to the top surface of thebody, the bearing layer extending peripherally at least partially beyonda periphery of the inner edge of the body, thereby forming a bearingoverhang.
 8. The method of claim 1, wherein the cut edge is at leastpartially chamfered.
 9. The method of claim 1, wherein said step ofmaking the sagittal cut comprises the step of making a substantiallylinear sagittal cut.
 10. The method of claim 1, wherein said step ofmaking the sagittal cut comprises the step of making a substantiallycurved sagittal cut.
 11. The method of claim 1, further comprising thesteps of: making a second sagittal cut in the articular surface of thetibia, the second sagittal cut extending from the posterior edge to theanterior edge of the tibia, at least a portion of the second sagittalcut extending obliquely to the dividing plane, whereby the obliquelyextending portion of the second sagittal cut extends away from thecentral compartment of the tibia; making a second transverse cut beneaththe articular surface of the tibia, the second transverse cut extendingfrom one of the lateral edge of the tibia and the medial edge of thetibia to the second sagittal cut and generally transversely to thelongitudinal axis of the tibia, thereby forming a second cut edgebetween the second transverse cut and the second sagittal cut; andresecting a second one of the medial and lateral compartments of thetibia along the second cut edge, while retaining at least a portion ofthe central compartment of the tibia and at least a portion of thespinous process to which the anterior cruciate ligament attaches.
 12. Amethod of preparing a tibia for knee arthroplasty to accommodate theanterior cruciate ligament, the tibia having an articular surface thatinteracts with a femur, a posterior edge, an anterior edge, a lateraledge, a medial edge, a central compartment, a lateral compartment, amedial compartment, a longitudinal axis located within a dividing plane,the dividing plane extending parallel to a sagittal plane and from theposterior edge to the anterior edge of the tibia, and a spinous processto which the anterior cruciate ligament attaches, the method comprisingthe steps of: making a sagittal cut into the articular surface of thetibia, the sagittal cut extending from the posterior edge to theanterior edge of the tibia; making a transverse cut beneath thearticular surface of the tibia, the transverse cut extending from one ofthe lateral edge of the tibia and the medial edge of the tibia to thesagittal cut and generally transversely to the longitudinal axis of thetibia, thereby forming a cut edge between the transverse cut and thesagittal cut, wherein a chamfered cut edge is formed between thetransverse cut and the sagittal cut; and resecting one of the medial andlateral compartments of the tibia along the chamfered cut edge, whileretaining at least a portion of the central compartment of the tibia andat least a portion of the spinous process to which the anterior cruciateligament attaches.
 13. The method of claim 12, wherein at least aportion of the sagittal cut extends obliquely to the dividing plane,whereby the obliquely extending portion of the sagittal cut extends awayfrom the central compartment of the tibia.
 14. The method of claim 12,wherein said step of making the sagittal cut comprises the step ofmaking a substantially linear sagittal cut.
 15. The method of claim 12,wherein said step of making the sagittal cut comprises the step ofmaking a substantially curved sagittal cut.
 16. The method of claim 12,further comprising the step of attaching an implant to the resectedcompartment of the tibia, the implant comprising: a body having aposterior edge, an anterior edge, an inner edge, one of a lateral edgeand a medial edge, a top surface, and a bottom surface, the posterior,anterior, inner, and one of the lateral and medial edges spanning thetop and bottom surfaces of the body, wherein the inner edge of the bodycorresponds to the sagittal cut and to the cut edge of the tibia, one ofthe lateral and medial edges of the body corresponds to one of thelateral and medial edges of the tibia, the posterior edge of the bodycorresponds to the posterior edge of the tibia, and the anterior edge ofthe body corresponds to the anterior edge of the tibia; and a bearinglayer coupled to the top surface of the body, the bearing layerextending peripherally at least partially beyond a periphery of theinner edge of the body, thereby forming a bearing overhang.
 17. Themethod of claim 12, wherein the cut edge is rounded.
 18. The method ofclaim 12, wherein an obtuse angle is formed between the transverse cutand the sagittal cut.
 19. The method of claim 12, further comprising thesteps of: resecting a second one of the medial and lateral compartmentsof the tibia; making a vertical bridge cut in the articular surface ofthe tibia, the vertical bridge cut extending across the centralcompartment from the medial compartment to the lateral compartment, andthe vertical bridge cut extending toward the anterior edge of the tibiawhen approaching the dividing plane; making a horizontal bridge cutbeneath the articular surface of the tibia, the horizontal bridge cutextending from the anterior edge of the tibia to the vertical bridge cutand generally transversely to the longitudinal axis of the tibia,thereby forming a bridge cut edge between the vertical bridge cut andthe horizontal bridge cut; and resecting a portion of the centralcompartment of the tibia along the bridge cut edge.
 20. The method ofclaim 12, further comprising the steps of: resecting a second one of themedial and lateral compartments of the tibia; making a vertical bridgecut in the articular surface of the tibia, the vertical bridge cutextending across the central compartment from the medial compartment tothe lateral compartment; making a horizontal bridge cut beneath thearticular surface of the tibia, the horizontal bridge cut extending fromthe anterior edge of the tibia to the vertical bridge cut and generallytransversely to the longitudinal axis of the tibia, wherein a chamferedbridge cut edge is formed between the vertical bridge cut and thehorizontal bridge cut; and resecting a portion of the centralcompartment of the tibia along the chamfered bridge cut edge.
 21. Animplant adapted for attachment to a tibia during knee arthroplasty toaccommodate the anterior cruciate ligament, the tibia having at leastone of a medial and lateral compartment resected along a cut edge formedbetween a transverse cut and a sagittal cut, the tibia also having alongitudinal axis located within a dividing plane, the dividing planeextending parallel to a sagittal plane, the implant comprising: a bodyhaving a posterior edge, an anterior edge, an inner edge, one of alateral edge and a medial edge, a top surface, and a bottom surface,wherein the posterior, anterior, inner, and one of the lateral andmedial edges span the top and bottom surfaces of the body, and whereinthe inner edge of the body corresponds to the sagittal cut and to thecut edge of the tibia, at least a portion of the sagittal cut extendingobliquely to the dividing plane.
 22. The implant of claim 21, whereinthe inner edge of the body comprises a pair of substantially linear,intersecting inner edge segments.
 23. The implant of claim 21, whereinthe inner edge of the body is curved.
 24. The implant of claim 21,further comprising a bearing layer coupled to the top surface of thebody, the bearing layer extending peripherally at least partially beyonda periphery of the inner edge of the body, thereby forming a bearingoverhang.
 25. The implant of claim 21, wherein the inner edge of thebody is at least partially chamfered.
 26. The implant of claim 21,wherein the body further comprises a second posterior edge, a secondanterior edge, a second inner edge, a second one of a lateral edge and amedial edge, and a bridge connecting the inner edges.
 27. An implantadapted for attachment to a tibia during knee arthroplasty toaccommodate the anterior cruciate ligament, the tibia having both amedial and a lateral compartment resected, the implant comprising: abody having a posterior edge, an anterior edge, a lateral edge, a medialedge, a first inner edge, a second inner edge, a bridge having an inneredge, a top surface, and a bottom surface, the bridge connecting thefirst and second inner edges, wherein the bridge, the first and secondinner edges, and the posterior, anterior, lateral, and medial edges spanthe top and bottom surfaces of the body, and wherein the bridge inneredge is at least partially chamfered.
 28. The implant of claim 27,wherein the first and second inner edges are at least partiallychamfered.
 29. The implant of claim 27, wherein the first and secondinner edges of the body extend away from each other from the posterioredge to the anterior edge of the body.
 30. The implant of claim 27,wherein the bridge inner edge is rounded.
 31. The implant of claim 27,further comprising a bearing layer coupled to the top surface of thebody, the bearing layer extending peripherally at least partially beyonda periphery of the inner edges of the body, thereby forming a bearingoverhang.
 32. An implant adapted for attachment to a tibia during kneearthroplasty to accommodate the anterior cruciate ligament, the tibiahaving both a medial and a lateral compartment resected, the implantcomprising: a body having a posterior edge, an anterior edge, a lateraledge, a medial edge, a first inner edge, a second inner edge, a bridgehaving an inner edge, a top surface, and a bottom surface, the bridgeconnecting the first and second inner edges, wherein the bridge, thefirst and second inner edges, and the posterior, anterior, lateral, andmedial edges span the top and bottom surfaces of the body, and whereinthe bridge inner edge approaches the anterior edge of the body betweenthe first inner edge and the second inner edge.
 33. The implant of claim32, wherein at least one of the first inner edge, the second inner edge,and the bridge inner edge is chamfered.
 34. The implant of claim 32,wherein the bridge inner edge is rounded.
 35. The implant of claim 32,wherein the bridge inner edge curves toward the anterior edge of thebody between the first inner edge and the second inner edge.
 36. Theimplant of claim 32, further comprising a bearing layer coupled to thetop surface of the body, the bearing layer extending peripherally atleast partially beyond a periphery of the inner edges of the body,thereby forming a bearing overhang.